1. Field of the Invention
This invention concerns improvements in and relating to analysis and screening, particularly but not exclusively, to the analysis and screening of materials from radioactive environments, such as those materials destined for decommissioing.
2. Present State of the Art
Any item or material which spends time within an active area of a nuclear facility may become contaminated by radioactive material from within that area. As a result of this, before the item can be removed and reused or recycled or disposed of, its level of contamination needs to be evaluated. Evaluation of the level of contamination is particularly significant as the higher category of waste the item is categorised as the greater the subsequent storage/disposal costs will be.
Specific detection. systems have been employed in the past to evaluate gamma sources present on or in conjunction within an item, with subsequent separate evaluation of beta sources through an additional piece of apparatus and with a still further piece of apparatus needed to evaluate the alpha contamination. Evaluation of the alpha contamination is particularly problematical as alpha particles can only be detected directly a very short distance from the source.
The present invention aims to provide a system and method of operation for analysis which is flexible in terms of the types of radiation source which can be investigated and evaluated.
The level of contamination an item possesses may vary considerably between individual items even where they have been exposed to substantially the same environment. As a consequence of this, there is minimal point in conducting detailed alpha and beta evaluation of an item if the gamma contamination subsequently proves to be substantial. To avoid this unnecessary time penalty and consequent cost, the present invention introduces a screening method whereby alpha contamination is only evaluated once gamma and/or beta contamination has been established to be below a certain threshold. This has Additional advantages in that the sensitive alpha detecting apparatus is not unduly contaminated by premature exposure of the system to an item very heavily contaminated with sources of radiation.
According to a first aspect of the invention we provide an instrument for monitoring one or more of alpha, beta and gamma emitting sources on an item or location, the instrument being provided with alpha and/or beta and/or gamma monitoring means for the item or location.
Preferably the item or location is in contact with a medium, alpha emissions generating ions in the medium. The instrument preferably has a detecting chamber, the detecting chamber being provided with one or more electrodes for discharging ions. The instrument is preferably further provided with means to monitor ions discharged and/or collected on the electrode (s). Preferably the instrument is provided with means for moving the medium from in proximity to with the item to the one or more electrodes.
The item(s) may be or include tools, pipes, pumps, filters, cables, beams, rods and the like. The locations may include surfaces in general, such as floors, walls, ceilings, soil, rubble, material on a conveyor, or parts thereof, as well as including parts of, or surfaces of, items, such as glove boxes, tanks, vessels and the like.
Preferably the item is mounted or supported so as to maximise the surface area exposed, for instance to the airflow.
The item or location may be introduced within the detecting chamber.
The medium may be a fluid, such as a liquid, but is normally a gas. The gas may be a mixture, such as air, or may be in substantially single gas form, such as argon.
The detecting chamber may comprise an elongate chamber. The detecting chamber may have a circular or rectilinear cross-section. Preferably the chamber is electrically conducting.
The detecting chamber may be provided with an inlet and an outlet, the electrodes being provided between the inlet and the outlet. The inlet and/or outlet may connect to the surrounding environment for the instrument, for instance to give an open circuit instrument.
Preferably means are provided within the instrument to remove extraneous ions and/or particulate matter in the medium. The ions and/or particles may be removed by a filter.
A filter may be provided downstream of the electrodes, preferably before the outlet. A filter is preferably provided in this way in a sealed system (a system in which the medium is re-circulated) and/or to remove particulate material.
Particularly in an open system, the filter may be provided between the inlet from the surrounding environment and the item or location. A filter may also be provided between the detecting chamber and the outlet to the surrounding environment.
The detecting chamber may be openable to introduce or remove an item or location.
Preferably the item or location is positioned upstream in the medium flow relative to the electrodes, where medium flow is used to move the ions.
The instrument may be provided with a single electrode. The instrument may be provided with a charge element or disc, such as an electret.
Preferably the apparatus comprises a plurality of electrodes, the electrodes being spaced from one another. The electrodes may be configured with a first outer electrode and a second outer electrode and none or one or more intermediate electrodes provided there between.
The electrodes are preferably arranged parallel to the direction of medium flow, such is airflow. Preferably the medium flow passes through the spacing between the electrodes.
One or more, and preferably all, of the electrodes may be planar. Preferably the electrodes are provided parallel to one another. Preferably the electrodes are provided in opposition, for instance, an outer electrode being opposed by one electrode, an intermediate electrode being opposed by two electrodes. the spacing between the electrodes is preferably the same between each pair of opposing electrodes. The spacing between the outer electrodes and the detector chamber is preferably the same as between opposing electrodes.
The electrodes may be continuous, such as a plate, or discontinuous, such as a grid.
An applied, preferably externally generated, potential may be employed. The electrical potential is preferably provided by an external power source. An electrostatic potential may be employed, for instance from a charged element, such as an electret.
Potentials of between 5 V and 3000 V, for instance between 10 V and 1000 V, may be provided.
The means for monitoring ions discharged on the electrode(s) may comprise electrostatic charge monitoring means. More preferably the means for monitoring ions discharged or collected on the electrode(s) comprise current indicating means and more preferably current measuring means. Preferably a single current measuring means is used. Preferably the combined current of all the electrodes connected to the current measuring means in measured. An electrometer, such as a ground referenced electrometer or floating electrometer may be used for this purpose.
The means for moving the medium may be a fan. Preferably the electrodes are provided between the medium moving means and the item or location. The fan may be of controllable, and preferably of variable, speed.
Preferably the medium moving means, such as the fan, can be inhibited so as to avoid medium flow. Preferably the medium flow means are inhibited in response to control signals. Preferably the control signals to the medium flow means are a function of the gamma and/or beta emission levels monitored. The medium flow means may be inhibited by stopping power supply to the fan and/or by presenting a physical barrier between the item or location and the medium flow means, most preferably between the item or location and the electrodes.
One or more discrete flow paths over a surface or surfaces of the item or a location may be provided. A pipe, for instance, may have an external flow path separated from an internal flow path by the material forming the pipe. Preferably means are provided for regulating the medium flow along one or more of the discrete paths. Detection of alpha sources on or in one more of the discrete paths alone may be provided by obscuring or inhibiting one or more of the other flow paths. Sealing means may be provided to inhibit flow along one or more of the flow paths, most preferably in a selective manner. Inflatable seals and/or iris seals and/or aperture seals may be provided.
The beta emissions may be inferred form on the measured alpha emissions and/or measured gamma emissions by means of calculation.
The beta emissions may be monitored directly by providing one or more beta detectors in association with the instrument. The beta detector(s) may be mounted within the detector chamber, preferably in proximity to the item or location. It is preferred that the beta detector(s) only be separated from the item or location by the fluid.
The beta detectors may comprise thin plastic scintillators connected to a photo tube or gas flow proportional counters.
The gamma emissions may be monitored directly by providing one or more gamma detectors in association with the instrument. The gamma detectors may be mounted in, on or outside the detector chamber. The gamma detectors may be part of and/or attached to and/or moveable relative to the detector chamber. A separate gamma detecting chamber may be provided.
The gamma detectors may comprise thick plastic scintillators or sodium iodide crystal scintillators or semi-conductor crystals, such as GeLi or intrinsic germanium.
According to a second aspect of the invention we provide a method for monitoring radioactive emitting sources on an item/location, the method comprising monitoring beta and/or gamma emissions from the item/location, and subsequently monitoring alpha emissions if conditions are met. The alpha emissions monitoring comprising contacting the item with a medium, alpha emissions generating ions in the medium, moving the medium to a one or more electrodes for discharging ions provided in a detector chamber, the method further comprising monitoring ions discharged and/or collected on the electrode(s).
Preferably the beta and/or gamma emissions from the item or location are monitored before the alpha emissions are monitored. Preferably the alpha monitoring means are physically isolated from the item or location during the beta and/or gamma monitoring stage. The physical isolation may be effected by inhibiting the medium flow means and/or by providing a barrier between the item or location and the electrodes.
Preferably the item or location is first monitored for gamma emissions. The monitoring period may be relatively short, for instance between 1 second and 1000 seconds, such as 5 seconds and 300 seconds. Preferably an, at least approximate, determination is obtained relating to the gamma emission level. The determination, such as activity may be compared with a threshold value. A relationship between the alpha emission level and gamma emission level may be derived. Different subsequent procedures may be applied depending on whether the threshold is exceeded, i.e. high contamination, or whether it is not reached, i.e. low contamination.
If high gamma detection is monitored the item or location may be classified accordingly and/or treated or decontaminated accordingly and/or disposed of or stored accordingly and/or further monitored. The item or location may be further monitored using the gamma detection means. The gamma monitoring may be performed for a longer period of time than the first monitoring. A period of between 60 seconds and 6000 seconds may be employed, The function of the gamma emissions achieved through further monitoring may be processed to give an indication, qualitative or quantitative, of the total level of radioactive material present and/or the total level of contamination and/or the type of contamination present.
If low gamma detection is monitored the item may be classified accordingly and/or further treated accordingly and/or disposed of or stored accordingly, but preferably the item or location is further monitored for alpha emissions.
Alpha emission monitoring may be effected by removing the inhibition on their monitoring. Ions produced by the alpha emissions are preferably conveyed to the electrode(s) by the medium flow means, preferably following a control signal from the gamma and/or beta monitoring stage. The function of the alpha emissions achieved alone and/or in combination with the gamma and/or beta emission function may be used to determine the further action for the item or location. The further action may involve disposal and/or recycling and/or decontamination and/or reuse.
The second aspect of the invention includes the features, options and possibilities set out elsewhere in this application, including the steps necessary to implement them.
According to a third aspect of the invention we provide an instrument for monitoring one or more of alpha, beta and gamma emitting sources on an item. or location, the instrument comprising 4 detecting chamber enclosing the item or location to be monitored, and one or more of alpha monitoring means, beta monitoring means. gamma monitoring means.
The third aspect of the invention includes the features, options and possibilities set out elsewhere in this application, including the steps necessary to implement them, with particular emphasis on the manner in which the alpha monitoring and/or beta monitoring and/or gamma monitoring means are provided.